RELATIONSHIP BETWEEN TREES, RAINFALL?

In 2009, CIFOR scientists Douglas Sheil and Daniel Murdiyarso summarized the scientific understanding of the relationship between forests and rainfall in an article in BioScience. At the time, the link between deforestation and declining rainfall was still uncertain. There were indications in the scientific literature that deforestation disrupted cloud formation and accentuated rainfall seasonality in areas that have distinct wet and dry seasons.

One of the mysteries at the time of Sheil and Murdiyarso’s article was how flat lowlands in the interior of continents maintained wet environments. If recycling is the key mechanism for rainfall reaching the interior of continents, then rainfall should decrease as distance from the coast increases. Indeed, in many places in the world, this phenomenon can be observed—except where there are extensive areas of natural forests.

A ‘BIOTIC PUMP’

In the mid-2000s, two physicists—Anastassia Makarieva and Victor Gorshkov of the Petersburg Nuclear Physics Institute—proposed a novel physical phenomenon to explain how tropical rainforests keep the interior of continents humid. In these regions, forests have higher evaporation rates than other vegetation types. As the humid air rises from forests into the atmosphere, the water vapor condenses. This decreases the volume of the air, and the air pressure plummets.

Because air flows from places of high pressure to those of low pressure, this decrease in pressure sucks additional dense air in, and so forests draw in moist air from elsewhere (for example, from over the oceans). This additional moist air rises and condenses in turn, generating a situation where a large proportion of the water condensing as clouds over wet areas is drawn in from elsewhere.

Makarieva and Gorshkov call this phenomenon a “biotic pump,” and they demonstrated it with data from the Amazon River Basin and the Congo River Basin in Equatorial Africa. Other scientists began looking at this phenomenon and provided additional evidence for the existence of a biotic pump. In 2012, Dominick Spracklen and others looked across the tropics and found that when air passed over extensive vegetation, it produced at least twice as much rain as air that passed over little vegetation.

Spracklen and others went further and integrated Makarieva and Gorshkov’s physical phenomenon into a climate model to see the effect of deforestation on rainfall. (Corrected) Makarieva and others went further and integrated this physical phenomenon into a climate model to see the effect of deforestation on rainfall. Because the primary air flow into the Amazon Basin is from the Atlantic Ocean, and because most of the deforestation occurs on the eastern and southern flanks of the Basin, there is cause for concern. Their simulation showed that continued deforestation in the Amazon Basin would lead to decreased rainfall.

RIVERS IN THE SKY

The idea that water flows around the atmosphere in observable pathways is not new; it was first proposed in an article by Reginald Newell and others in Geophysical Research Letters in 1992. Several studies have confirmed these and show, in addition, that these “aerial rivers” are responsible for the rainfall in southeastern Brazil. Contrary to surface rivers, these aerial rivers gain water from the vegetation as it pumps water out of the soil and lose it through rainfall.

Several groups have been working on this phenomenon during the past decade, and our understanding of the importance of these aerial rivers has grown. In particular, we now understand how these massive flows of water through the atmosphere relate to rainfall around the South American continent.

To cite one study, Josefina Moraes Arraut and others from the Brazilian Institute of Space Studies showed that as the air masses move across the Amazon, enhanced by the biotic pump, they eventually encounter the Andean Cordillera, where they veer south and eventually east to bring humidity from the Amazon Basin to southeastern Brazil and northern Argentina. Thus, maintaining the biotic pump in the Amazon is essential for ensuring water delivery to southeastern Brazil.

WHAT SHOULD POLITICIANS DO?

The governor of São Paulo expressed skepticism over the role of the Amazonian deforestation in the drought affecting his state in an article that appeared in The Wall Street Journal last month. Yet the science is clear, and it goes beyond simple correlation among observations: The mechanisms of water circulation between the Amazon Basin and the southern regions east of the Andes are well established.

As deforestation in the Amazon continues, rainfall in the southern part of Brazil will continue to be affected. The Amazonian forest will continue to lose its ability to regulate the climate and ensure a flow of water to the southeastern part of the country. Additionally, deforestation in the Brazilian Amazon will affect northern Argentina, so the problem has an international dimension.

Politicians need to balance development objectives and environmental concerns, but in this case it is fairly clear that the two go hand in hand. Thus, there appear to be two options for wise action.

On the one hand, politicians can decide to stop the problem at its root cause by decreasing Amazonian deforestation and promoting rehabilitation of degraded forest in order to maintain the atmospheric circulation patterns. A second possibility is to integrate expected shortfalls of rainfall into planning and adapt the economic systems of the south to accommodate more frequent droughts. This means improving water storage and distribution while reducing waste. A combination of these two approaches is probably warranted.

There is a third option: One could ignore the problem until it goes away. Climate is variable, and this drought will eventually end. However, it is very likely that this is not an isolated event, and the science suggests that there is more to come.

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Thanks for writing this article. I was wondering if this biotic pump also covers regions in Bolivia and Paraguay since they border northern Argentina.
“Additionally, deforestation in the Brazilian Amazon will affect northern Argentina, so the problem has an international dimension.”

Brazil’s leading climate scientist, Dr Antonio Donato Nobre, is calling for a wartime effort to restore the Amazon and reverse the drought effects caused by its deforestation which equates to 184 million football fields worth of rainforest:

…So concerned is Dr Nobre about what is happening that he believes only a virtual war effort can save the rainforest. His battle plan – with ignorance the first enemy to overcome − has five steps:

1. Popularising forest science: On the basis that knowledge is power, scientific facts about the role of the forest in creating a friendly climate, and the effect of deforestation in leading to an inhospitable climate, must become common knowledge.

2. Zero deforestation: The harm deforestation does to human beings and the economic losses it causes should be compared with that of tobacco, Dr Nobre argues. When Brazil introduced a new Forest code that scaled back protection, the consequences of changed land use on the climate were never discussed by the politicians. While economic growth and market demand create pressures that leads to deforestation, planning weaknesses foster the invasion and occupation of forested areas − and all these loopholes must be sealed urgently.

3. An end to fires, smoke and soot: Using fire as a tool for clearing land is a deeply ingrained habit that must be stopped. The fewer sources there are of smoke and soot, the less damage will be done to the formation of clouds and rain, resulting in less damage to the green-ocean rainforest.

4. Recover and regenerate forest: Stopping deforestation is not enough to reverse threatening climate trends. “We must regenerate, as widely as possible, all that has been changed and destroyed,” Dr Nobre says. Reforestation on such a scale implies a reversal of land use in vast areas that are now occupied − difficult in the current scenario − and land zoning technologies will be needed.

5. Governments and society need to wake up: In 2008, when the global financial bubble burst, governments around the world took just 15 days to decide to use trillions of dollars of public funds to save private banks and avoid what threatened to become a collapse of the financial system. The climate crisis has the potential to be immeasurably worse than any financial crash, yet still there is procrastination − despite the abundance of scientific evidence and of viable, creative and appealing solutions.

Good points. But, other scientists have said that increased soot from fires can slow AGW by screening out incoming sunlight energy. And some ecosystems require fire, including some in tropical S.A., such as the Pantanal.

Forest fires are a net contributor to global warming, by way of transforming enormous quantities of stored plant-based carbon into CO2 and thereby eradicating these carbon sinks in favor of GHG-increasing modes of agricultural production. Make no mistake, this isn’t a net-zero natural process that’s taking place in the tropics. Whereas some ecosystems do require fire for health, most tropical rain forests rarely see much in the way of fire, and certainly not to the extent that is being deliberately introduced by humans. And these lands are almost entirely utilized for practices that further contribute to undesirable anthropogenic climate change.

Regarding the effects of smoke and soot, that’s a very complex process that has many causes and effects, but forest fires have never (to my knowledge) been shown to be anything but net contributors to warming. The type of aerosols you’re referring to that are helping to keep the earth a degree-or-so cooler than we would be in their absence, are mostly contributed by industrial processes, not forest fires. In any event, though, your comment seems to be arguing that we should fight the effects of pollution with more pollution, reminiscent of 18th century medical practices.

This was predicted thirty years ago, when cattle ranchers and other criminals began logging and clearing operations.
We humans deserve every catastrophe and calamity that befalls us, if we continue our pre-atomic/industrial age ways of thinking.
I’ve got a six-pack and a lawn chair ready for when the fun starts. No camera, though. Witnesses would say pix were photoshopped, I reckon.

CIFOR advances human well-being, equity and environmental integrity by conducting innovative research, developing partners’ capacity, and actively engaging in dialogue with all stakeholders to inform policies and practices that affect forests and people. CIFOR is a CGIAR Research Center, and leads the CGIAR Research Program on Forests, Trees and Agroforestry (FTA). Our headquarters are in Bogor, Indonesia, with offices in Nairobi, Kenya, Yaounde, Cameroon, and Lima, Peru.